As an adjunct to the diagnosis and treatment of disease, the medical industry commonly employs various types of particle flow systems, such as that diagrammatically illustrated in FIG. 1, to analyze particles or cells in a patient's body fluid (e.g., blood cells). To this end, a carrier fluid (e.g., saline) stream 1, containing particles/cells 2 of a centrifuged blood sample stored in a blood sample holding chamber 3, is directed along a flow channel 4 through a restricted flowcell `measurement` aperture 5 of a flowcell 6 into a receiving chamber 7. The flowcell measurement aperture 5 is sized and configured to allow the particles to be counted one at the time as they pass through the flowcell, and includes a pair of electrodes 8 and 9, to which a DC electrical field for measuring the size or volume of each particle and an RF field for measuring the density of each particle passing through the flowcell aperture 5 are applied.
In particular, the dimensions of the flowcell measurement aperture 5 define a "steady state" flowcell characteristic impedance R.sub.a, which may be represented by a single capacitance and resistance value at the frequency of interest. As particles pass through the flowcell measurement aperture 5, they introduce changes in the resistance of the flowcell in proportion to their size or volume. These changes in aperture resistance are reflected as DC voltage pulses at the electrodes 8 and 9, and can be measured directly.
In addition, the density or opacity of a blood cell or particle is reflected as a change in the reactance of the flowcell aperture, and has been conventionally measured by coupling the electrodes 8 and 9 in parallel with the resonance (LC tank) circuit of an associated RF oscillator-detector circuit 10. This change in reactance of the flowcell causes a corresponding change in the operation of the RF oscillator, which can be measured by means of an RF pulse detector/demodulator. For an illustration of non-limiting examples of U.S. patent literature detailing such conventional oscillator-based flowcell RF detector circuits attention may be directed to the U.S. patents to Coulter et al, U.S. Pat. No. 3,502,974; Groves et al, U.S. Pat. No. 4,298,836; Groves et al, U.S. Pat. No. 4,525,666; and Coulter et al, U.S. Pat. No. 4,791,355.
Now although an RF oscillator-based flowcell measurement circuit of the type generally shown in FIG. 1 is effective to provide an indication of both size and density of each blood cell, it suffers from a number of problems which are both costly and time-consuming to remedy. One fundamental shortcoming is the fact that the particle detection mechanism was originally designed as and continues to be configured as a tube-based RF Hartley oscillator circuit. This potentially impacts circuit availability, as the number of manufacturers of vacuum (as well as gas filled) electronic tubes continues to decline.
In addition, the effective lifetime of a newly purchased and installed tube in the Hartley oscillator is not only unpredictable, but experience has shown that the effective functionality of most tubes within the Hartley oscillator--detector circuit is very limited, (even though a tube tester measurement shows a tube to be good). At best a tube can expect to last somewhere in a range of three to nine months--and typically involves on the order of two repair/maintenance service calls per year per flowcell.